Such an illuminating unit is disclosed, for example, in publication DE 102004004772 A1.
LEDs or thermal emitters are based on the principle of spontaneous emission such that the radiation emitted from the respective luminous means is emitted in all spatial directions, without a preferred direction, in the absence of measures used for beam shaping. Consequently, for the purpose of beam shaping of the radiation emitted from the luminous means, use is made in illuminating units of optical elements that are based as a rule on reflection or refraction of the light, such as mirrors, lenses or combinations of such elements.
So called TIR lenses that are based on the principle of total internal reflection, for example, are known for beam shaping an LED light source. In order to attain good beam shaping, such lenses generally have a diameter to height ratio of typically approximately 2:1.
Furthermore, Fresnel lenses are frequently used for beam shaping an LED light source. Because of the comparatively large focal length of a Fresnel lens, it can be difficult to bring the latter close enough to the LED in order to capture the flat emerging light.
Suitability of holographic optics for incoherent white light is known from the publications I. Weingartner, K. J. Rosenbruch, “Incoherent polychromatic imaging with holographic optical elements and systems I”, Optik, Vol. 57 (1980), p. 103 ff. and S. Prakash, N. Deva Das, “Imaging in white light using compact two hololens systems”, J. Optics, Vol. 26 (1995), p. 267 ff.